Bivalent europium activated barium octaborate luminescent material

ABSTRACT

BIVALENT EURONIUM ACTIVATED BARIUM OCTABORATE PHOSPHORS, IN WHICH THE GREATER PART OF THE LUMINESCNET ENERGY IS RADIATED BETWEEN 380 AND 440 NM. IN RESPONSE TO ULTRAVIOLET EXCITING RADIATION, HAVE UTILITY IN MERCURY VAPOR DISCHARGE LAMPS.

pail 2%, k 97i BLASSE ETAL 3,798 1% BIVALENT EUROPIUM ACTIVATED BARIUM OCTABORATE LUMINESCENT MATERIAL Filed May 9, 1969 zno Win06 3&0

INVENTOR. GEORGE BLASSE JAAP DE VRiES nited States Patent 3,575,879 BIVALENT EUROPIUM ACTIVATED BARIUM OCTABORATE LUMINESCENT MATERIAL George Blasse and Jaap de Vries, Emmasingel, Eindhoven, Netherlands, assignors to US. Philips Corporation, New York, N.Y.

Filed May 9, 1969, Ser. No. 823,347 Claims priority, application Netherlands, May 10, 1968, 6806648 Int. Cl. C09k 1/66 US. Cl. 252301.4 3 Claims ABSTRACT OF THE DISCLOSURE Bivalent europium activated barium octaborate phosphors, in which the greater part of the luminescent energy is radiated between 380 and 440 nm. in response to ultraviolet exciting radiation, have utility in mercury vapor discharge lamps.

The invention relates to a low-pressure mercury vapour discharge lamp including a luminescent material provided on a support, which luminescent material can be excited by ultraviolet radiation. Furthermore the invention relates to such a material.

In many photochemical document-copying systems a document is copied by irradiating the master and directing the radiation which has been reflected or transmitted to a piece of paper which is sensitive to this radiation and contains substances which can be decomposed by the radiation so that a copy of the master document is obtained possibly after further treatment, for example, fixing.

For efiicient use of the reproduction papers it is of course desirable to have a radiation source which emits a strong radiation at those wavelengths to which the paper is most sensitive.

A requirement generally imposed on the reproduction papers to be used is that the substances which are sensitive to radiation are converted as little as possible by normal daylight. This of course facilitates working with these papers and also imposes few requirements on the storage thereof. Since normal daylight contains comparatively little ultraviolet radiation the best combination is apparently a paper which has a maximum sensitivity below 400 nm. and a radiation source which emits a strong ultraviolet radiation.

As already stated above the master to be copied must transmit or reflect the radiation. It has been found that many documents are made from paper which transmits and/ or reflects ultraviolet radiation comparatively poorly. In view of the contradictory requirements for documentcopying machines a compromise must therefore be made; it is therefore preferred to use light-sensitive papers the maximum sensitivity of which lies between 380 and 440 nm. and a radiation source having a maximum of the emitted radiation between these two values.

Document-copying machines generally use as radiation sources mercury vapour discharge lamps including a luminescent layer provided on a support which layer converts a great part of the ultraviolet radiation produced in the mercury vapour discharge into radiation of longer wavelength. Thus, as described above, the maximum emitted radiation energy must preferably be in the wavelength range between 380 and 440 nm. This is, for example, the case with the very frequently used substance, calcium tungstate. The conversion etficiency of the ultra violet radiation of the mercury vapour discharge into the radiation between 380 and 440 nm. is, however, comparatively small for this substance, because the emission spectrum is very wide and hence much radiation energy is emitted at wavelengths outside this range. In addition, the absorption spectrum of most light-sensitive papers is con- Patented Apr. 20, 1971 siderably narrower than this range. As a result of these two causes only a comparatively small part of the total radiation energy emitted by the calcium tungstate is efficiently used by the sensitive paper.

A further luminescent substance, which is very frequently used, is a lead-activated silicate of strontium, barium and magnesium. The emission spectrum of this substance is not very wide when excited by the ultraviolet radiation of a mercury vapour discharge, and hence is more suitable for adaptation to the absorption spectrum of a radiation-sensitive paper; the maximum emission of this substance lies, however, at 355 nm. and is therefore less suitable to be transmitted or reflected by the paper of most documents. In spite of this fact the substance is very frequenly used on account of the narrow emission band and the strong radiation.

A low-pressure mercury vapour discharge lamp according to the invention includes a luminescent material provided on a support and is characterized in that this luminescent material is an alkaline earth borate activated by bivalent europium and having the following composition wherein 0 x+y 0.l5 O yQOS 0.003 51150.05

A luminescent material which is represented by the above formula can be excited satisfactorily by ultraviolet radiation which is emitted by a low-pressure mercury vapour discharge lamp and the material then shows an emission spectrum in which the greater part of the luminescent energy is radiated between 380 and 440 nm. Since the conversion efiiciency is also very high, namely considerably higher than that of calcium tungstate and approximately as high as that of the above-mentioned silicate, a lamp according to the invention is more suitable for use in document-copying apparatus in combination with available radiation-sensitive types of paper having a maximum absorption within this range, because all requirements as set above are now simultaneously satisfied. In addition it has been found that the substances according to the invention have a good temperature dependence, that is to say, their conversion efiiciencies decrease only slightly as the temperature increases. They are thus very suitable for use in high-power low-pressure mercury vapour discharge lamps whose wall temperature assumes a high value during operation.

As is evident from the above given formula, a luminescent material according to the invention consists of a barium octaborate activated by bivalent europium wherein the barium may be partly replaced by strontium and/or calcium. In fact, tests have shown that the luminescent properties do not substantially change when replacing small quantities of barium by strontium and/or calcium. Thus, for example, the maximum in the emission spectrum for all materials according to the invention lies at the same wavelength, namely at approximately 400 nm.

For contents of strontium and calcium outside the above given limits substances are obtained which are less suitable for practice, because the quantum efficiency and the light output of the luminescent materials then become too small. Pure barium octaborate activated by bivalent europium is preferred.

It is to be noted that luminescent materials are known from United States patent specification No. 2,920,046 which have a fundamental lattice according to the formula aMeO.bB O wherein Me is at least one of the metals calcium, strontium, barium and cadmium and wherein the ratio a/b has a value between /2 and The substances mentioned in the said patent specification are activated by manganese together with lead and/or bismuth. The known substances do not, however, emit in the blue part, but in the green and yellow parts of the spectrum.

The amount of bivalent europium may be varied between the above-mentioned limits, but is preferably chosen A mixture was made of the substances indicated in column III of the table in the quantities indicated in column IV. Since the boric acid partly evaporates during the reaction, an excess of H BO of approximately 20% relative to the stoichiometric quantity is always used.

to be between 0.008 and 0.02. In fact, the highest radia- 5 The mixture was heated at approximately 600 C. for tion eificiency is found in this range. 4 hours. After cooling of the firing product obtained it In addition to the above-mentioned advantages of the was ground and again heated at approximately 750 C. luminescent materials according to the present invention for 4 hours. In both cases the heat treatment took place in it is to be noted that the substances are only slightly 1O amixture of nitrogen and hydrogen. The ratio of nitrogen affected by oxidation. This is of great importance in the to hydrogen is then not critical; a ratio of, for example, manufacture of mercury vapour discharge lamps, because 20:1 was found to be quite usable. The hydrogen serves they are then often exposed for a short period to a heat for reducing the trivalent europium to bivalent europium. treatment in air at a fairly high temperature, for ex- After cooling subsequent to the second heat treatment ample, 600 C. Such a heat treatment is necessary, for the reaction product obtained was ground and sieved, example, when an organic binder is used which is to be if necessary. The product was then ready for use. removed later on by a heat treatment. The quantum efficiency q is given in column V of A special advantage of a luminescent material accordthe table in percent. The percentages indicated show the ing to the present invention is that a light-reflecting layer conversion eificiencies of the absorbed exciting quantums. of titanium dioxide may fruitfully be used in the mercury To obtain a measure of the light output of the luminescent vapour discharge lamps according to the invention. The materials the quantum efficiency is to be multiplied by use of such a layer in low-pressure mercury vapour disthe absorption factor. The absorption factor is supposed charge lamps is known. This light-reflecting layer, which to be equal to l-r, wherein r represents the reflection is provided between the support of the luminescent matefactor. rial and the luminescent material, very strongly reflects The absorption factor is indicated in column VI. The visible radiation emitted by the luminescent material. In relative light output (r.l.o.) of the luminescent materials this manner it is, for example, possible to manufacture as indicated in column VII are then derived from the lamps in which the visible radiation from one side of quation! the lamp is accentuated. Such a. principle would be very r.l.o.:q. (l-r) gg 2; {amps l' g The wavelength of the maximum of the emission band W er mmescen m er a S are o for the different materials was found to be always approxithe emitted radiation energy lies for a considerable part mately 400 nm i g fi s f 3 i i i 2 All measurements were made using an exciting radiaw a ove-men1one nown su s ances an par icu a1 y tion which had a wavelength of 254 nm. the silicate of barium, strontium and magnesium, the use 30 In the drawino of a E l f i FIG. 1 diagrammatically shows a low-pressure mercury i f 1 p Sma u vapour discharge lamp according to the invention; v10 i i p 3 g; g t FIG. 2 is a graphic representation of the radiation 1 3 ca e g t is re acts P Stan 40 intensity of the substance of Example 2 of the table, and 8 no {3 5 0W a Wavelength of apProxfmately of two known substances, as a function of the wavelength,

h ectlon of the anatase modlficatlon FIG. 3 is a graphic representation of the variation of ten s s 1g t y urther, namely down to a waveleng h of the radiation intensity of the substance of Example 2 with approximately 380 nm. temperature In a mercury vapPur filscharge P FIG. 1 shows a low-pressure mercury vapour discharge accordmg to lnventlon a reflecting tltanlum f f lamp which includes an envelope 1. Electrodes 2 and 3, l y y be f1'111tfu 11Y pal'tlcularly tltanlum dlOXlde between which the discharge takes place during operam the anatase modificat on, because at least the greater hon f the lamp are provided at the ends of the lamp part of theradiation emitted from the luminescent mate- The inner Side f the envelope 1 Which is made f f r131 15 Wlthm reflection range of f tltalflllm tiloXlde example, glass, is coated with a luminescent layer 4 which Some embod ments of the present invention will now contains a luminescent material according to the present he described with reference to one table, the following invention. The luminescent substance is provided on the example and the accompanying diagrammatic drawing. envelope 1 in any conventional manner.

I II III IV V VI VII Composition firing mixture Example Formula in grams 4; 1r r.l.o.

BaCO; 1.970

1 BamosEllmoaBso s {Eugos 0. 009 0.00 36 H3303 6.000 BaCO; 1, 960

2 BaomEllnmBsoia {B11203 0.018 60 0.70 42 HaBOs 6.000 BaCO 1. 940

3 UJB UBAZ E B {121120 0.036 f 45 0.85 38 4 namsriionutmnion iggg; 8 5%; i is 0. 34

11 130 6.000 BaCO; 1. 6 0

5 nattisroiinurmmos {$5,383 3:33} 40 0. 75 0 HsBOa 6. 000 BaCO 1.860

0 au.al o.0s uu.orBs is {$32 2 8 2 40 0.75 30 HsBOa 6.000 BaCO 1.760 SrCO 0. 074

7 Ban.a S1n Cfl Eu mB 013 (M003 0-050 40 0-75 30 The broken-line curve a in the graph of FIG. 2 shows the spectral energy distribution of the knOWn leadactivated silicate of barium, strontium and magnesium and the broken-line curve 11 shows the spectral energy distribution of the known calcium tungstate. These curves are shown for comparison both for the spectral distribution and for the intensity of the luminescent radiation. The maximum intensity of the curve a is fixed at 100. The curve 2 relates to the material of Example 2 0f the table. As is clearly evident from the drawing, luminescent materials which are used in lamps according to the present invention have a more favourable location of the maximum emission in the spectrum as compared with the known silicate, while they have considerably higher peak values and a narrower emission range as compared with the known calcium tungstate.

The curve 2 in the graph of FIG. 3 shows the temperature dependence of the radiation intensity of the material of Example 2. The temperature is plotted in C. on the abscissa. The maximum intensity is fixed at 100. The figure shows that the borates according to the invention have a very good temperature dependence and still show an intensity of the luminescent radiation at approximately 220 C. which is equal to half the maximum value. It may be noted for comparison that already at 75 C. the intensity of the known calcium tungstate has decreased to half the value at room temperature.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What We claim is:

1. An alkaline earth borate luminescent material activated by bivalent europium and having the composition Ba Sr Ca Eu B O wherein O y QOS 0.003pll05 2. The luminescent material of claim 1 which material has the composition Ba Eu B O wherein 0.003 p0.05.

3. The luminescent material of claim 1 wherein 0008 1 002.

References Cited UNITED STATES PATENTS 3,423,325 1/1969 Wanmaker et al. 252-301.4

3,431,216 3/ 1969 Chenot 252301.4

FOREIGN PATENTS 6408082 1/1966 Netherlands 252301.4

TOBIAS E. LEVOW, Primary Examiner R. D. EDMONDS, Assistant Examiner 

